Put on the pants and mark the desired length with pins or chalk
Remove the pants and measure the hem allowance you want to keep
Turn the pants inside out
Fold the fabric up to the marked line
Press the fold flat with an iron
Fold the raw edge under again to hide it inside the hem
Pin the folded hem in place
Thread a needle with matching thread and knot the end
Use a blind stitch or slip stitch to sew the hem by hand
Keep stitches small and even
Sew all the way around the leg
Tie off the thread securely when finished
Trim any excess thread
Press the hem again for a neat finish